1. Field of the Invention
The present invention relates to the field of printed circuit boards. More particularly, the present invention relates to a land pattern architecture used to facilitate more accurate electrical testing of a printed circuit board after its assembly and a corresponding separate electrical contact testing method.
2. Description of Art Related to the Invention
For many years, electronic systems have been designed to include one or more printed circuit board ("PCB"). Each PCB includes a (i) substrate board, being a number of insulation and metal layers selectively patterned to provide metal interconnect lines (referred to herein as "traces"), and (ii) a plurality of components mounted on a surface of the substrate board and interconnected through the traces.
Currently, there are two primary techniques of attaching components to the PCB; namely, through-hole mount technology ("THMT") and surface mount technology ("SMT"). Unlike THMT components in which its leads are inserted into plated holes in the substrate board and soldered in place, SMT components have terminations or leads (generally referred to as "electrical contacts") that are soldered directly to the surface of the substrate board. The driving force behind the use of SMT components involves their reduced package size and assembly simplicity.
The SMT components are attached to the substrate board in accordance to land patterns. A "land pattern" includes a plurality of metalized mounting pads placed on the surface of a substrate board in order to establish electrical connections between a SMT component and at least one trace connected to the mounting pads. The selected orientation of these mounting pads is such that one mounting pad is dedicated to one electrical contact of the SMT component. More specifically, each electrical contact of the SMT component is coupled to a unique mounting pad through a solder joint which is typically formed when the PCB undergoes reflow soldering to attach the SMT components.
After assembly of the PCB, it is necessary to test each solder joint in order to verify that all of the SMT components are properly attached and have proper electrical connections to the PCB. In other words, testing is performed to check for both solder open defects and solder short defects. A "solder open" defect is a condition where there is too little or no solder at an intended solder joint to form a proper connection. A "solder short" defect is a condition where two adjacent electrical contacts, which are not intended to be connected, are connected due to excess application of solder.
Several verification techniques are now being used to determine whether a PCB is defect free. These verification techniques include (i) visual inspection and (ii) electrical testing, both of which may be performed separately or in combination. For visual inspection, a person inspects each side of the PCB with the unaided eye or magnification. For SMT components, defects (especially solder open defects) are difficult to visually detect because most of the solder joint is obstructed from view.
For electrical tests, there are two well-known types of tests; namely, Automated Test Equipment ("ATE") testing and Functional testing. For ATE testing, the PCB is mounted on a test fixture having a number of spring probes. These spring probes establish simultaneous contact with various access pads of the PCB. An "access pad" is an electrical connector to a one or more mounting pads. The access pad is normally located on one or both surfaces of the PCB. By applying a set amount of current into a selected spring probe in contact with a first access pad and measuring current at a second access pad, the solder joints that are disposed on the mounting pads connected to the first and second access pads can be checked for solder open defects and solder close defects. Of course, a completed circuit must be formed between the first access pad and the second access pad in order to perform such measurements. The combination of the land pattern with circuitry used for electrical testing (e.g., is referred to herein as "land pattern circuit".
For Functional tests, the PCB is not connected to specialized test equipment such as ATE test equipment. Rather, it is connected and operated in a similar fashion to how the PCB will be used in an intended electronic system. For example, a PCB acting as a motherboard could be functionally tested by connecting a power supply, a keyboard, a mouse, a monitor, a hard disk drive and a floppy drive. Upon being powered on, the components and solder joints of the PCB are tested during use. The problem is that it is difficult to write a test program that will test every single joint on the PCB during a brief test period. More likely, there would be many potential solder open defects that will go undetected.
It is evident that there are certain components that are more difficult to test for solder open defects than others. For example, electrically isolated SMT components, such as SMT connective components (e.g., sockets, headers), are particularly difficult to test for solder open defects as illustrated in FIGS. 1-2. SMT socket 100 includes a receptacle area 105 providing access to a number of electrical contacts 110.sub.l -110.sub.n ("n" being a positive whole number, n&gt;1) attached to a PCB 120 through surface mount technology. Each electrical contact 110.sub.11 -110.sub.n is coupled to PCB 120 through solder joints 111.sub.1 -111.sub.n between electrical contacts 110.sub.1 -110.sub.n and mounting pads 112.sub.1 -112.sub.n. The mounting pads 112.sub.1 -112.sub.n have a one-to-one correspondence with electrical contacts 110.sub.1 -110.sub.n. Each electrical contact of the SMT socket 100 is electrically isolated from the other electrical contacts so that each solder joints 111.sub.1, . . . , and 111.sub.n cannot be tested separately. Thus, without a SMT component inserted into the receptacle area 105 of the SMT socket 100, a circuit for measuring current propagation is not completed, precluding current propagation as indicated by arrows in FIG. 2.
Currently, there is no conventional technique for electrically testing an empty SMT socket 100 of FIG. 1 because each electrical contact is electrically isolated from every other electrical contact. Therefore, to test the SMT socket 100, a SMT component 180 must be placed within the receptacle area 105 as shown in FIG. 2. Such placement of the SMT component 180 establishes a completed circuit between a pair of electrical contacts. As shown by arrows, the current propagation path originates at a first spring probe 130 and propagates through a first access pad 140, a trace 150, a first mounting pad 160, a first solder joint 170 disposed on the first mounting pad 160, the SMT component 180, a second solder joint 171, a second mounting pad 161, a trace 151, a second access pad 141 and a second spring probe 131. The requirement of implementing a component before electrically testing the SMT socket 100 poses a number of disadvantages.
One disadvantage is that the installation and removal of components in a high volume, low cost production environment increases the overhead costs realized in constructing PCB assemblies (e.g., increased labor costs, decreased production rate, etc.). Another disadvantage is that installation and removal of a component from the SMT socket may damage the socket. The damaged socket would go undetected because removal of the component occurs upon completing electrical testing of the PCB. As a result, a defective PCB may be accidentally installed into an electronic system to be sold to an end user or shipped to the manufacturer of the electronic system to be subsequently returned as defective. A further disadvantage is that installation and removal of components from the SMT socket may damage the component itself. This may cause inaccurate test results for later tested PCB boards as well as increase manufacturing costs due to continued replacement of damaged components or testing of the component before each test.
In view of the foregoing, it would be quite beneficial to develop a land pattern which would enable testing of an interconnection of a single electrical contact at a time. The unique land pattern (referred to herein as a "split-pad land pattern") involves the substitution of a plurality of mounting pads for a single mounting pad previously assigned to each electrical contact and the addition of more access pads connected to the bottom side of the PCB to support the additional mounting pads.